Intermittent stalling

[Eyvind Spangen]

When I press the clutch right after "pushing" the engine with boost
and more than half throttle, it will often stall and require
restarting. Sometimes it stumbles a bit upon restarting. This never
happens under normal conditions, but I have noticed the idle seems to
go very low and then back up. 

Can this be the bypass valve malfunctioning? I can hear it working, a
small "psscch" sound when letting off the gas pedal after boost..

-- 
E. Spangen
'90 Audi 200 quattro 20v